The present invention relates to active devices such as amplifiers with a very low phase error.
There are several applications that require a stable amplifier with a very low phase error. For example, to achieve precise power factor measurement (i.e., measurement of the phase shift between the voltage across and current through a component), the phase shift in a small resistance connected in series with the component is measured. The measurement system requires an amplifier with a low phase error, independent over time and temperature variation.
For background information, reference is made to amplifiers with active feedback described by J. Wong in EDN Sep. 17, 1987, and by A. A. Khan in IEEE Transaction on Instrumentation and Measurement Vol. 41, No. 4, p 555; and to amplifiers that use a feedback trim to adjust the phase, as described by R. A. Pease in EDN, Aug. 20, 1977, p. 138.
It is an important object of the invention to provide apparatus and technique for amplifying a signal without creating a phase error. The apparatus is an active device with a restricted band width selected to include a desired frequency band of operation. The device includes one or more cascaded amplifiers, each with a local feedback loop that establish a high forward gain, and an RC network arranged to maintain the phase shift at 180.degree.. The phase error is independent of the time and temperature fluctuations in the amplifier itself.
In one aspect, the invention features an amplifying apparatus having a nearly zero phase error at desired operating frequencies. The apparatus comprises at least one amplifier, including local feedback loop, formed by a network, constructed and arranged to exhibit a high forward gain at the operating frequencies. The amplifier and the network are constructed and arranged to provide substantially 180 degree phase shift between an input voltage and an output voltage at the operating frequencies. The apparatus also comprises a signal input node adapted to introduce the input voltage to the inverting input of the amplifier, a signal output node adapted to obtain the output voltage from the output of the amplifier, and a resistive global feedback loop that connects the signal input and signal output nodes and controls the closed loop gain of the amplifying apparatus.
In another aspect, the invention features an amplifying apparatus exhibiting a nearly zero phase error at desired operating frequencies. The apparatus comprises three cascaded amplifiers including local feedback loops, formed by a network constructed and arranged to exhibit a high forward gain, at the operating frequencies, wherein each amplifier with its respective local feedback loop is arranged to form a low pass filter. The network has, at the desired frequency band, substantially 180 degree phase shift between an input voltage and an output voltage of the amplifying apparatus. The apparatus also comprises a resistive global feedback loop arranged to connect the output of the third amplifier and the input of the first amplifier. The global feedback loop is adapted to control the closed loop gain of the apparatus.
In another aspect, the invention features an amplifying apparatus having a nearly zero phase error at desired operating frequencies. The apparatus comprises an amplifier, having inverting and noninverting inputs and an output, constructed and arranged to operate as a low pass filter with a high gain and to have a cut-off frequency substantially below the operating frequencies. The apparatus also comprises a signal input node adapted to introduce an input voltage to the inverting input, a signal output node adapted to obtain an output voltage from the output of the amplifier, and a capacitor-resistor network constructed and arranged to provide, in connection with the amplifier, substantially 180 degree phase shift between the input voltage and the output voltage at the operating frequencies. A resistive global feedback loop connects the signal input and signal output nodes and is adapted to control the closed loop gain of the amplifying apparatus.
The capacitor-resistor network is connected either between the signal input node and the negative input of the amplifier, or, alternatively, the capacitor-resistor network is connected between the output of the amplifier and the signal output node.
Preferred embodiments of these aspects of the invention may include one or more of the following features.
At least one amplifier further comprises a phase compensation network arranged to increase the frequency range over which the 180 degree phase shift is achieved.
The local feedback loop of at least one amplifier is formed by a capacitor and a resistor connected in parallel adapted to establish a cut-off frequency substantially above the desired frequency band.
At least one amplifier further comprises an input network of a capacitor-resistor loop and a resistor loop connected in parallel acting as a phase lead network to provide a phase margin.
At least one amplifier further comprises a phase compensation network arranged to increase a frequency range over which the 180 degree phase shift is achieved.
The amplifying apparatus further comprises a high frequency noise reduction stage, cascaded to the three amplifiers, constructed and arranged to reduce noise of frequencies substantially above the operating frequency band.
The capacitor resistor network comprises a first passive low pass filter with a cut-off frequency substantially below the operation frequency, and
a second passive low pass filter with a cut-off frequency substantially above the operation frequency.
The network also comprises a high frequency attenuator adapted to reduce the forward gain at high frequencies.
Each amplifier may comprise a capacitor adapted and arranged for external compensation.
The desired frequencies of operation are in the range of 50 Hz to 60 Hz.
In another aspect, the invention features an amplifying apparatus having a nearly zero phase error at a desired frequency band. The apparatus comprises at least two cascaded amplifiers with local feedback loops with a network constructed and arranged to exhibit a high forward gain wherein the local feedback loops are constructed and arranged to establish a predetermined restricted pass band containing frequency. The network has, at the operating frequencies, substantially 180.degree. phase shift between an input voltage and an output voltage. The apparatus also comprises a resistive global feedback loop arranged to connect the output of the second amplifier and the input of the first amplifier, wherein the global feedback loop is adapted to control the closed loop gain of the amplifying apparatus.
Preferred embodiments of this aspect of the invention may include one or more of the following features.
Each of the amplifiers with the local feedback is loop is constructed and arranged to form a high pass filter.
Each of the amplifiers with the local feedback loop is constructed and arranged to form a low pass filter.
The overall gain may be one or larger than one.
The operation frequency is in the range of 50 Hz to 60 Hz.